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    var arr = ['关羽', '刘备', '曹操', '赵云', '黄忠', '张飞', '姜维'];
    for (var i = 0; i < arr.length; i++) {
      console.log(arr[i]);
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    // 1.数组的长度使元素个数，不要和索引号混淆
    // 2.arr.length 动态检测数组元素的个数
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